1. Field of the Invention
The present invention relates to an art on combination body of a shim and a cam for use in a valve train mechanism of an internal combustion engine and driving mechanism of a valve train of a high-pressure fuel injection pump of an automobile or the like for improving the durability of mechanism parts by reducing sliding friction resistance and wear loss.
2. Description of the Prior Art
Recently, as an effective means for reducing a mechanical loss of automobile engine to improve fuel efficiency, an art for reducing friction loss of the engine mechanical portion has been proposed. Among others, reducing a friction loss of the shim and cam in the valve train mechanism which has a high sliding speed and a high load is very effective in improving the fuel efficiency.
On the other hand, in an in-cylinder direct fuel injection engine in which gasoline or light oil is injected directly into a combustion chamber, the shim and cam for use in the reciprocating mechanism for obtaining a fuel feeding pressure are sliding parts which slide under a strict condition. Reducing the friction loss of these parts is an important technology for improving the performance and durability of the fuel injection pump. Particularly, in the fuel injection train for the in-cylinder direct fuel injection engine, it has been demanded that by converting a conventional roller follower type with a cam and a cam roller made of steel used as intermediaries to a slipper follower type in which the shim and cam materials are improved, reductions in the number of parts, size and weight thereof are achieved, thereby reducing production cost.
And, the conversion from the roller follower type to the slipper follower type is achieved by minimizing a friction loss between members in a frictional interface. As regards the friction-loss, if the reciprocating mechanism portion exists in lubricant environment, generally it has been considered that the minimum gap between the members or the minimum oil film thickness and characteristic of the sliding surface affects/affect the sliding characteristic and friction-loss.
The aforementioned friction-loss is quantified by the following formula (1).
F=A{xcex1Sm+(1xe2x88x92xcex1)St}xe2x80x83xe2x80x83(1)
where, F: friction-loss, A: sliding area, xcex1: oil film fracture area ratio, Sm: shearing strength of a member in case where a mating member is in direct contact, St: shearing strength of oil film, xcex1Sm: friction-loss (friction-loss under boundary lubricating condition) in a case where no oil film exists, and (1xe2x88x92xcex1)St: friction-loss (friction-loss under fluid lubrication) in a case where an oil film exists completely. Here, in order to decrease the friction-loss F, it is necessary to increase the term of the friction-loss under fluid lubrication and decrease xcex1 because usually Sm is larger than St.
Further, in order to maintain a complete fluid lubrication state, it is important to control the nature of the sliding surface of the mating sliding member. The oil film parameter xcex9 indicating a degree of lubrication is quantified by the formula (2). Increasing this value xcex9 is effective for maintaining the fluid lubrication.
xcex9=hmin/(Rrmsl2+Rrms22)xe2x80x83xe2x80x83(2)
where, hmin: minimum gap between mating sliding members or minimum oil film thickness, Rrms1: mean-square roughness of the surface of one of the sliding members, and Rrms2: means-square roughness of the surface of the other of the sliding members. Therefore, it is understood that if the surface roughness of the mating sliding members is made fine, it is effective for maintaining the fluid lubrication.
On the basis of the above mentioned technical background, in JP-A-7-98052 there is proposed an art on the combination body of the shim and cam, in which the sliding surface roughness of the shim composed of ceramic containing silicon nitride or SIALON is made less than Rz0.1 xcexcm in 10-point mean roughness, and the surface of the cam made of cast iron is chill hardened and phosphate coating film is formed thereon.
According to this conventional art, the surface roughness of the cam is improved during a running-in operation or initial phase of the operation so as to reduce a friction-loss at a portion to be subjected to boundary lubrication, thereby the sliding characteristic between the shim and cam being improved. As a result, cam shaft driving torque can be largely reduced. Further, it is explained therein that since the surface roughness of the cam can be improved during the running-in operation, the friction-loss can be reduced even if any special ultra precision finish processing is not carried out on the surface of the cam having a complicated shape, and this is very effective also in economic viewpoint.
In case where the slipper follower type is employed, it is necessary to choose the material of the shim and cam capable of suppressing as much as possible the boundary lubrication which is likely to occur if the surface pressure goes high even in the lubricant environment. That is, it is necessary to choose the material having Sm characteristic which unlikely undergoes plastic deformation by a direct contact with a mating member or deterioration of the surface roughness due to corrosion or the like by various impurities mixed in lubricant. Particularly, the shim having a flat sliding surface is easy to have a fine surface roughness by ultra precision processing even if a material having a high hardness is chosen, so that means for maintaining this hmin characteristic stably should be added. Further, if the surface of the cam member is provided with means for accelerating the hmin characteristic, it is effective for maintaining the fluid lubrication. The present invention proposes an art for solving these problems.
In the combination body of the shim and cam for use in the reciprocating mechanism portion, the shim is made of a material having a hardness higher than the sliding surface of the cam and the sliding surface of the shim is finished in a range of Rz0.07-0.2 xcexcm in terms of 10-point mean surface roughness. As to the sliding surface of the cam, fluid lubricating condition is maintained by utilizing the open pores existing in the surface of the cam member.
For the shim member, ceramic composed of silicon nitride or SIALON is chosen, and for the cam member, iron-base alloy sintered body is used and the pores dispersed inside the sintered body are utilized. The sliding surface of the cam is treated by etching with acid so as to control the open pores so that the area ratio thereof is 2-6% and the maximum diameter is less than 50 xcexcm, thereby improving the oil film holding function.
The shim member has the surface of the steel material coated with a coating film of ceramic composed of nitride or carbide of Cr and Ti or a coating film of diamond or DLC. The cam member is made of the iron-base alloy sintered body and the cam member hardness is increased by heat treatment. As a result, the wear resistance of both is improved.